1. Field of the Invention
The present invention relates to an excitation device and method for downhole seismic testing using the same, and more particularly, to an excitation device for downhole seismic testing, which is installed on the ground to generate seismic waves with efficiency and accuracy and allows separation into individual units for convenient carriage and secure installation, and a method for downhole seismic testing using the same.
2. Description of the Related Art
For efficient management of water that is essential for human life and industrial development, structures, such as dams, reservoirs, seawalls, banks, and the like, can be built not only with reinforced concrete, but also in the form of an embankment structure through construction of earth materials such as soil, sand, rock, and the like.
The crest of the embankment structure constructed of the earth materials is generally paved with asphalt, concrete or other special material having flexibility, or is protected by blocks having various shapes and sizes not only for the purpose of construction of a road or sightseeing, but also for the protection of an embanked body from various factors, such as erosion, weathering, water of infiltration, and the like. Since the embankment structure for water conservation or management is very closely related to variation of the underground water level in view of stability and functions thereof, observation holes for observing the underground water level are formed at major points on the crest of the embankment structure to allow periodic observation of variation in the underground water level.
Large scale and deep excavation works are frequently performed in cities for foundation works for large buildings or structures or for works for construction of underground spaces, and various kinds of measurement are periodically performed with respect to the surrounding ground during excavation works to determine stability of neighboring buildings. In this case, various observation holes for observing the underground water level are formed corresponding to places on the ground near an excavation site, the majority of the surface of which has been already paved. Since the effect of excavation work on the surrounding ground can be determined at various points through periodic measurement of the variation of the underground water level, the measurement of the variation of the underground water level is considered a crucial factor.
Generally, excavation work or the embankment structure for water conservation or management is planned or designed in consideration of the aim or performance guarantee term in view of geotechnical engineering or hydraulics. Even in this case, however, stability of a target structure or the surrounding ground and structures can change during construction of the structure or over time due to abnormal variation of material or design factors, unpredicted inner or outer factors, and the like.
From this point of view, it is necessary to perform periodic and continuous measurement of the embankment structure or the conditions of the surrounding ground near an excavation site. Particularly, in the embankment structure for water conservation or management or the ground near an excavation site in which the underground water level is higher than an excavation base, there can be a gradual or rapid variation in engineering characteristics of the ground, which acts as an inner construction material, due to variation of the underground water level or secondarily induced factors. Change in characteristics of the ground material is directly related to local or overall stability of a structure. Particularly, dynamic stiffness at a small strain level, that is, body wave velocity as a characteristic of quantitative seismic waves is considered one of the most important material characteristics in the related art.
Since behavior with respect to effective stress corresponding to behavior of pore fluid is very important for ground materials, shear wave velocity (VS) is considered a major factor in the body wave velocity composed of compressional wave velocity (VP) and shear wave velocity (VS). From this point of view, evaluation of target ground materials must be systemized based on a useful ground engineering technique through periodic and continuous measurement of the shear wave velocity.
Nevertheless, periodic and continuous measurement and evaluation of the shear wave velocity for most ground materials which require stability has never been taken into consideration. Further, in evaluation of overall stability with respect to a target structure, non-periodic seismic testing under limited conditions, such as non-destructive seismic testing on the ground surface, is generally used to confirm distribution of the shear wave velocity in a material.
Generally, among the techniques for determining distribution of a shear wave velocity according to an increase in depth, a borehole seismic test method is performed through construction of boreholes and has higher reliability than a surface wave test method which is performed on the ground surface. In particular, downhole seismic testing as illustrated in FIG. 1 has higher economic feasibility and efficiency than crosshole seismic testing and is thus actively applied to ground engineering.
Referring to FIG. 1, for downhole seismic testing, with a hexahedral excitation source 10 such as a log placed on the ground surface, a borehole 40 is formed vertically from the ground surface according to an increase in excavation depth and at least one receiver 50 for detecting excavation is prepared. The downhole seismic test is performed in-situ by obtaining a seismic wave signal generated on the ground surface while changing the location of the receiver 50 according to variation in excavation depth. Here, when confirming excitation from an initial motion detector 20 connected to the excitation source 10, a dynamic signal detector 30 placed on the ground obtains the seismic wave signal from the receiver 50 inside the borehole 40.
Seismic wave signals according to depth as obtained in-situ are analyzed through several steps to determine distribution of the shear wave velocity according to excavation depth (Chang-guk Sun, Hong Jong Kim, Jong-hong Jung, Gyung-ja Jung, 2006, “Synthetic Application of Seismic Piezo-cone Penetration Testing for Evaluating Shear Wave Velocity in Korean Soil”, Geophysical exploration, Volume 9, No. 3, pp. 207-224). Then, variation in conditions and stability of a target structure can be evaluated based on quantitative variation according to temporal and spatial differences of the shear wave velocity.